Sea urchin sperm undergo the acrosome reaction when contacting the egg jelly layer. This releases the acrosomal process which contains the egg-binding protein. This process binds to proteinaceous sperm receptor sites on the egg vitelline layer. The sperm bore through the vitelline layer, the plasma membranes of sperm and egg fuse and the cortical granules of the egg break down, releasing a protease which destroys the sperm receptor sites. I propose to isolate and characterize the component(s) of the egg jelly which cause the acrosome reaction. The proteins of the sperm membrane which bind the active jelly component will be isolated by affinity chromatography and characterized. The interaction of the purified jelly component and the sperm membrane protein will be studied to explain the mechanism of the acrosome reaction. The egg-binding protein(s) from the acrosomal process of sperm will be isolated, characterized and antiserum prepared to study its location on the sperm. Eggs will be used as an assay system for egg-binding protein and also as affinity surface for purifying the protein. The sperm binding sites on the vitelline layer will be isolated by solubilizing them with dithiothreitol or by isolating intact layers. Components will be fractionated by preparative isoelectric focusing column chromatography, or by binding to lectins and assayed to determine which is the sperm-binding protein. The binding of purified sperm receptor sites with egg-binding protein will be studied in detail. Striking similarities exist between sea urchin and mammalian fertilization. Knowledge gained by studying sea urchin sperm-egg binding will be applicable to fertility control in mammals.